RFeB system sintered magnets were discovered in 1982 by Sagawa (one of the present inventors) and other researchers. The magnets have the characteristic that most of their magnetic characteristics (e.g. residual magnetic flux density) are far better than those of other conventional permanent magnets. Therefore, RFeB system sintered magnets are used in a variety of products, such as driving motors for hybrid or electric automobiles, battery-assisted bicycle motors, industrial motors, voice coil motors (used in hard disk drives or other apparatuses), high-grade speakers, headphones, and permanent magnetic resonance imaging systems.
In the RFeB system sintered magnet, the grains of the main phase (R2Fe14B) are surrounded by an RL-rich phase having a higher Nd content than the main phase and a B-rich phase having a higher B content than the main phase. Among these phases, the main phase and RL-rich phase become easily oxidized when they are in contact with oxygen or water. The RL-rich phase is particularly easy to be oxidized. If the RL-rich phase is oxidized, a brittle region made of an oxide, hydroxide or similar compound of RL is formed, which may cause discoloration or rust in a region near the surface of the RFeB system sintered magnet and consequently cause the main phase grains in the surface region to come off.
Patent Literature 1 discloses the technique of performing a fluorination treatment on the surface of a produced RFeB system sintered magnet to form a protective layer made of a fluoride of rare earth R on that surface. This protective layer produces an anti-corrosion effect for preventing the RFeB system sintered magnet from being corroded due to oxidization. However, this method requires the additional process of forming the protective layer.
Patent Literature 2 discloses a method in which a protective layer is formed on the surface of an RFeB system sintered magnet by using a grain boundary diffusion method.
In the grain boundary diffusion method, a powder or some other form of material containing a heavy rare-earth element RH (Tb, Dy or Ho) is made to be in contact with the surface of the RFeB system sintered magnet and heated, whereby RH atoms are diffused through the grain boundaries of the RFeB system sintered magnet into the inner regions. However, RH are rare and expensive elements, and furthermore, they unfavorably decrease the residual magnetic flux density Br and the maximum energy product (BH)max of the RFeB system sintered magnet. With the grain boundary diffusion method, it is possible to overcome these problems while at the same time improving the coercivity, by introducing RH into only the regions near the grain boundaries in the RFeB system sintered magnet. As just noted, the grain boundary diffusion is originally a treatment process aimed at improving the coercivity. However, according to the method described in Patent Literature 2, the single process of heating the RFeB system sintered magnet, with a metallic powder containing Ni and/or Co with RH placed in contact with its surface, can produce both the effect of improving the coercivity and the anti-corrosion effect by a layer which remains on the surface of the RFeB system sintered magnet after the heating process for the grain boundary diffusion is completed.